It is well known that effects of purification of raw material are greater and much purer metal can be appropriately produced in an electron beam melting furnace since the degree of pressure reduction can be set lower compared to that in a vacuum arc melting furnace.
Among electron beam melting furnaces, there are many cases in which a furnace for purification, called a hearth, is arranged therein. By melting raw material in the hearth, impurities contained in the raw material can be effectively removed, thereby enabling purification of the raw material.
In an electron beam melting furnace, a target is heated and melted by emitting an electron beam emitted from an electron gun arranged on the ceiling part of the melting furnace. Since an electron beam that is emitted from an electron gun has a property of traveling straight, by bending the electron beam by a deflection coil and by setting intensity of the electron beam, the target can be reliably heated and melted.
However, inside the hearth in the electron beam melting furnace, the target is held while being melted by heating with an electron beam, and metal is also vaporized from the target location. In a case in which the vapor enters into the electron beam route, the electron beam and the metallic vapor interfere with each other, and as a result, there may be a case in which the direction of the electron beam is changed, and the electron beam cannot be emitted in the required direction that is set. Furthermore, in addition to this situation inside of the electron beam melting furnace, there may be a case in which direction of the electron beam is changed to an undesirable direction by noise that entered from the outside. Furthermore, it is well known that the electron beam and the deflection coil to bend the electron beam are easily influenced by an external magnetic field, and in that case, there may be a case in which the electron beam is bent to an undesirable direction, and thus, improvements for these problems has been required.
In the case in which the electron beam is spread in an unintended direction as described above, for example, the wall of the hearth in which molten metal is held may be heated, and in a case in which this heating is not considered for a long time, the hearth may be undesirably damaged.
To solve such problems, a technique is known, in which temperature distribution of a part heated by an electron beam is measured and intensity of the electron beam is adjusted based on the measured value (See Patent Document 1). However, there is no disclosure regarding correction of unintended displacement of location of the electron beam spot.
Furthermore, a means is known, in which erroneous emission of an electron beam is detected by detecting characteristic X rays that are generated in a case in which the electron beam emitted from an electron gun erroneously emits a mold (See Patent Document 2).
However, in this method, since erroneous operation can be detected after the erroneous emission of an electron beam, the mold cannot be prevented from being damaged, and the hearth may be undesirably significantly damaged.
As mentioned above, a technique is desired, in which an electron beam emitted from an electron gun can be prevented from being undesirably displaced and the electron beam can be reliably emitted to a target to be heated.
The Patent Documents are follows:    Patent Document 1: Japanese Unexamined Patent Application Publication No. Hei 05 (1993)-192747    Patent Document 2: Japanese Examined Patent Application Publication No. Hei 06 (1994)-003727
An object of the invention is to provide a technique in which an electron beam is emitted accurately to a pre-set location of a molten metal pool formed in a hearth or a mold arranged in an electron beam melting furnace.
The inventors have researched in view of the above circumstances, and they have found that a region to be heated of a molten pool formed in a hearth or a mold can be accurately controlled as follows: two pieces of information are compared; one piece of information is a location information of high electron beam intensity spot of molten pool in the hearth or the mold arranged in the electron beam melting furnace to which an electron beam is certainly emitted that is output by an image sensor, and the other piece of information is a location information of a region to be emitted from an electron beam gun that is preliminarily recorded in a controlling means of the electron beam, and a location of the electron beam spot is controlled so that the difference between the two pieces of information becomes minimal. Thus, the present invention has been completed.